1. Field of the Invention
The present invention relates generally to an internal combustion engine exhaust system and more specifically to an exhaust gas purifying device for reducing particulate matter emissions.
2. Description of the Prior Art
FIG. 1 shows an exhaust system which is disclosed in JP-A-58-51235 and which includes a trap for removing particulate matter (e.g. minute carbon particles) from the exhaust gases before they are released into the ambient atmosphere.
In this prior proposed arrangement the particulate matter which is contained in the gases exhausted from the combustion chambers of an internal combustion engine 1 into an exhaust conduit 2, are collected in a trap 3. This trap includes a heat resistant filter element (not shown in this FIGURE) which separates the particulate matter from the gases content of the engine exhaust.
In order to prevent the exhaust gas back pressure from rising excessively due to the accumulation of a large amount of particulate matter in the trap, it has been proposed to regenerate the trap from time to time.
More specifically, the engine includes an induction passage 5 in which a butterfly type throttle valve 6 is disposed. A lever 7 is connected to the shaft of the valve 6 and operatively connected with a diaphragm type vacuum motor 8 by way of a link 8a.
A solenoid valve 9 which controls communication between a vacuum pump 10 and a vacuum chamber 8b of the vacuum motor 8 is operatively connected with a control unit 15. This latter mentioned unit is connected with a fuel injection pump 11 and arranged to receive a load indicative signal produced by a load sensor 13 and an engine speed sensor 13. In this instance both of the sensors are associated with the pump 11 as shown. The control unit 15 is also connected with an induction pressure sensor 14 in a manner to receive a signal indicative thereof.
The control unit 15 is arranged to determine the timing with which the trap 3 should be regenerated based on either time or distance travelled. Upon such a determination being made, the control unit determines, based on the outputs of the above mentioned sensors, if the engine is operating in a predetermined mode wherein an excess of air is being inducted into the engine.
Given that the engine is operating in the requisite mode, the control unit issues a signal to the solenoid which induces the throttle valve to partially close. The degree to which the throttle valve is closed and induction is throttled is feedback controlled based on the output of the induction pressure sensor 14. This feedback control is such as to adjust the duty cycle of the solenoid driver signal in a manner to establish an essentially constant negative induction pressure (e.g. -200 mmHg) in the induction manifold downstream of the throttle valve 6. This is done to prevent the power output of the engine being excessively degraded and to prevent the operation of compression-ignition type engines being de-stabilized to the point where the accumulation of particulate matter exceeds the re-burning rate.
It is to be noted that generally the trap cannot be regenerated at low speed/low load and idling modes of operation even if the induction system is throttle in the above mentioned manner.
To overcome this problem it has been proposed to additionally throttle the exhaust system. However, this measure does not lend itself to stable engine operation and in some cases can lead to the situation wherein the back pressure reaches levels of 2-3 kg/cm.sup.2 and results in the exhaust throttle valve sticking and/or or undergoing mechanical deformation.